Because of a statistically demonstrable relationship that appears to exist between the size of the late replicating X chromosome in circulating leukocytes and the occurrence of premalignant and malignant lesions in the female reproductive tract and breast, skin fibroblasts and leukocytes are cultured, challenged with SV40 virus and scored for changes in the karyotype, level of transformation efficiency via T antigen induction and focus formation scores. Results have shown a positive correlation between the two methods of test - namely, X chromosome aberrations and transformation efficiencies. Confirmation is currently being sought via computer measurements of chromosomes, chromosomal banding, and viral transformation studies. Positive correlations could lead to diagnostic testng and recognition of otherwise undetected malignancy or premalignancy in the general female population.